This project concerns the molluscan bivalve hinge ligament and its relationship to the mantle isthmus cells responsible for its formation. The distribution of the aragonite crystals into noded prisms and pseudo-prisms has been described. It has been determined that the crystals are twinned and surrounded by a protein sheath which is distinct from the bulk organic phase of the ligament. These sheaths have been isolated and chemically characterized. The ligament secreting cells are specialized for their function; the cytoplasm contains abundant rough endoplasmic reticulum and prominent Golgi complexes. The function of the cells in the mineralization process is under investigation. The cytochemistry of the cells and the ultrastructure of the ligament-isthmus cell junction will be studied. The ultrastructure of the hinge ligament and mantle isthmus cells in bivalves belonging to the family Pectinidae, which have a noncalcified ligament, will be described. The purpose of this project is to determine what differences exist in the calcified and noncalcified ligament forming systems. This comparison will elucidate important structural factors necessary to the mineralization process, and may suggest important structural alterations which must develop in normally noncalcified tissues prior to pathological mineralization.